Embodiments of the invention relate generally to a circuit and method for protecting insulated gate bipolar transistors (IGBTs) against short circuit conditions.
An IGBT is a switching transistor used to permit power flow in when it is on and to stop power flow when it is off. It is a solid-state device and has no physical moving parts. Instead of opening and closing a physical connection, the IGBT is operated by applying voltage to a semiconductor component, which changes its properties to create or block an electrical path. IGBTs are commonly used as switches, e.g., in chopper and frequency converter applications to control and convert electrical power by switching electrical devices on and off at predetermined instances.
IGBTs usually are designed to reliably handle circuit currents under normal as well as estimated overload conditions. Generally, to a certain extent, IGBTs are capable of withstanding fault conditions caused by anomalous operations. However, under a fault or short-circuit condition, an IGBT may be subjected to a very high surge current. The short-circuit current may be four times its rated current, resulting in both high voltage and high current simultaneously being applied to the IGBT. The IGBT under the fault or short-circuit condition may be subjected to a power loss with consequent increased thermal stress, which may damage the IGBT. Therefore, protecting IGBTs against short circuit conditions is important.
Existed methods manage to reduce a peak current at the beginning of the short circuit fault by dynamically reducing resistance of a turn-on resistor in a gate driver for the IGBT. However, the short-circuit fault current at a steady state is still not reduced, resulting in the considerable thermal loss during the fault.
As another method to protect against short circuit conditions, U.S. Pat. No. 6,104,149 describes using a shunt resistor in series with the emitter of the IGBT to reduce the overall short-circuit fault current. However, the power loss of the IGBT cannot be significantly reduced through the described method. Instead, adding such a series-connected shunt resistor will increase the power loss, resulting in additional junction temperature increase of the IGBT die if the shunt resistor is embedded into the IGBT die. Moreover, the actual gate voltage cannot be accurately controlled in the described method.
For these and other reasons, there is a need for embodiments of the present invention.